1. Field of the Invention
This invention relates to an image forming apparatus based on electrophotographic process applied to printers, facsimile apparatuses, copiers and so forth and, more particularly, to an image forming apparatus, which comprising a drum-like or belt-like photo-sensitive member accommodating exposure means, and in which the development is effected substantially simultaneously with the exposure of the photosensitive member with the exposure means.
2. Description of the Prior Art
Heretofore, a commonly termed Carlson process electrophotographic apparatus is well known, in which various processmeans for exposure, development, transfer, cleaning (i.e., removal of residual toner), discharging and charging are disposed around the outer periphery of a photo-sensitive drum for performing image formation in a predetermined electro-photo-graphic process.
In such apparatus, however, the individual process means have to be disposed independently around the photo-sensitive drum outer periphery. In addition, high voltage is required not only for the charging of the photo-sensitive member but also as a development bias, thus inevitably complicating and increasing the scale of the construction.
To overcome these drawbacks, there has been proposed an image forming apparatus (disclosed in, for instance, Japanese Laid Open Provisional Patent Application No. 153957/83), which comprises a photo-sensitive drum formed by alamination of a transparent conductive layer and a photo-conductive layer on the cylindrical transparent support and accommodating exposure means for generating an optical output in correspondence to image information. The optical output of the exposure means is focused through a convergence lens to effect exposure of the above photo-conductive layer and latent image formation thereon. Simultaneously with or soon after the exposure, the latent image is developed into a toner image with a toner support member facing the photo-sensitive drum, the toner image being then transferred onto recording paper by a transfer roller or like transfer means.
This type of image forming apparatus usually employs variuos proposed exposure means. One such exposure means (as disclosed in Japanese Laid Open Provisional Patent Application No. 14283/88) used a LED head, in which a large number of LED elements are arranged in a row extending in the axial direction of the drum and selectively turned on in corresponding to image information. Another exposure means (as disclosed in Japanese Laid Open Provisional Patent Application No. 280773/87) uses a liquid crystal head, which has a liquid crystal shutter disposed between a light source and a convergence lens and on-off controlled to form an exposure image. A further exposure means (as disclosed in Japanese Laid Open Provisional Patent Application No. 280773/87) adopts a technique of using an electroluminescence head, which has a group of electroluminescence elements arranged in a row.
With the above structure, in which the LED unit is accommodated as the exposure means in the drum, it has been impossible to provide an apparatus using a photo-sensitive drum having a diameter of 50 .phi. or below, particularly 40 .phi. or below. With recent development of small width LED head array, however, it has been made possible to provide an apparatus using a photo-sensitive drum with a diameter of 40 .phi. or below, more suitably 30 .phi. or below.
In such apparatus, however, a brushing contact region is formed in a zone, in which the photo-sensitive drum and a development sleeve face each other, and the three process steps of charging, exposure and development are effected on a drum portion corresponding in position to brushing contact region. Therefore, with the reduction of the diameter of the drum and sleeve the brushing contact region is reduced proportionally, making it difficult to obtain the charging, exposure and development smoothly.
With reducing drum diameter the angle of the wedge-like spaces formed on the opposite sides of the position, at which the drum and sleeve are closest to each other, is increased, thus making the brushing contact region to be correspondingly narrower. In addition, since the toner layer carried on the development sleeve has small thickness, the more the angle of the wedge-like spaces, the more it is difficult to form the brushing contact region accurately, that is, the nip width and the toner density are the more unstable.
Particularly, with reducing drum diameter the line drum speed has to be increased in a geometrical series fashion in inverse proportion to the drum diameter in order to obtain a paper feed speed, which satisfies the function of a printer or a facsimile apparatus producing 10 or 6 A-4 size prints per minute. Increasing the line drum speed leads to increased unstability of the brushing contact region, and unstable and narrow brushing contact region has direct adverse effects on the toner density and sharpness and stability of the image formation.
In the meantime, the toner used for the apparatus described above has to be a conductive toner because charge injection is made from the side of the development sleeve. Using a conductive toner, however, the efficiency of transfer of toner image onto paper is inferior.
To overcome this drawback, it is suitable to use insulating toner rather than conductive toner for the developer. In the case of using the insulating toner, however, it is impossible to carry out a charging step of causing charge injection from the development bias, and frictional charging is the sole resort. Therefore, it is difficult to obtain smooth charging.
To obviate this difficulty, a technique (disclosed in Japanese Laid Open Provisional Patent Application No. 214781/88) has been proposed, which uses a blend toner obtained by mixing a conductive magnetic toner with a resistivity of 10.sup.4 to 10.sup.9 .OMEGA..cm and a frictional charging type high resistivity toner with a resistivity of 10.sup.14 .OMEGA..cm or above and capable of being charged to have a predetermined number of poles by frictional charging, the resistivity of the blend toner being set to 10.sup.5 to 10.sup.10 .OMEGA..cm.
However, even though the resistivity of the above conductive toner is in a semiconductive range of 10.sup.4 to 10.sup.9 .OMEGA..cm, or the resistivity of the blend toner is again in a semiconductive range of 10.sup.5 to 10.sup.10 .OMEGA..cm, with such semi-conductive range materials, the resistivity is generally varied depending on the electric field intensity and increases exponentially with reducing electric field.
Therefore, even when a developer with the resistivity in a semiconductive range of 10.sup.5 to 10.sup.10 .OMEGA..cm, for instance, is used, with the charging of the photo-sensitive member up to the neighborhood of the development applied voltage, the developer present in the development space becomes high resistivity developer. In this case, smooth injection of charge into the photo-sensitive member, i.e., charging, can not be obtained.
This means that with the use of the above semiconductive range toner, of either non-blend or blend type, it is impossible to provide enough charge for sufficient erasing of the preceding image at the time of the re-charging. This results, in the subsequent development step, in a ghost generation due to history (or preceding latent image) formed by the exposure to image or a fog due to lack of the re-charging of the photo-sensitive member.
The above drawback due to a sudden increase of the developer resistivity in a low electric field range, is more pronounced in the case of using a photo-sensitive member of a-Si or like material, the resistivity of which is low compared to the conventional OPC or other organic semiconductors. Particularly, with the photo-sensitive member using a-Si, the resistivity of which is low compared to the above semiconductor range developers, the charging defectiveness is enhanced to cause readier generation of the fog or ghost.